In order to surface-harden such parts, it is known to apply a nitriding treatment (sometimes accompanied by carburizing, in which case the term “nitrocarburizing” is often used). In fact, the concept of nitriding encompasses both nitriding alone, in a bath with a very low cyanide content (typically less than 0.5%), as well as nitrocarburizing in the case of cyanide contents above this threshold. The term “nitriding” will be used for both these types of treatment hereafter.
This nitriding can be carried out from a gas phase or a plasma phase or from a liquid phase.
Liquid phase nitriding has the advantage of allowing significant hardening over a thickness of several microns within a period of just a few hours, but has the significant drawback of involving the use of molten salt baths, at temperatures of the order of 600° C. (or even more), in practice containing cyanides, in combination with cyanates and carbonates (the cations are in practice cations of alkali metals, such as lithium, sodium, potassium, etc.). In practice the cyanates decompose to form in particular cyanides, carbonates and nitrogen which is thus available to diffuse into the part to be nitrided. Due to the consumption of cyanates and enrichment with carbonates, regeneration of the baths must be provided by introducing additives making it possible to return their cyanide and cyanate contents to ranges guaranteeing their effectiveness. Hereafter, the contents of the baths are expressed in percentages by weight.
As is well known, the use of cyanides is dangerous to operatives as well as to the environment, such that for decades it has been sought to minimize the quantity of cyanides to be used in the processes for nitriding mechanical parts made of steel in molten salt baths.
Thus, in 1974-75, it was proposed to seek to minimize the cyanide content of nitriding baths, in particular by avoiding the toxic products at the time of the regeneration, (FR-2 220 593 and FR-2 283 243, or U.S. Pat. No. 4,019,928, or also GB-1 507 904); in fact these documents mentioned, without special comment, an alkaline chloride content which can range up to 30% (without however giving any example, for nitriding, comprising more than 5% by weight NaCl in a bath also containing 64% potassium cyanate, 16% potassium carbonate, 11% sodium cyanate and 4% sodium cyanides). It was considered that baths with a low cyanide content had to be essentially constituted by potassium or sodium cyanates, potassium or sodium carbonates, with more potassium than sodium (which made it possible to lower the temperature of the salt baths); the objective was to reduce the cyanide content to not more than 5%, or even 3%); the reduction in the cyanide content had to be compensated for by cyanates; there was no particular explanation of the role of the chlorides apart from the fact that, in the carburizing baths, barium chloride is a melting flux.
Previously (see the document GB-891 578 published in 1962), it had been mentioned that nitriding-carburizing baths could contain alkaline chlorides, which made it possible to economize on the cyanides and cyanates, the price of which is much higher, or to reduce the melting point; this document related to salt baths containing from 30% to 60% cyanides and taught the maximizing of the content of n-cyanates relative to the isocyanates (there were no chlorides in the example described).
There had also been mentioned (see the document GB-854 349 published in 1960), carburizing baths (used at temperatures from 800° C. to 950° C.) containing, by weight, from 35% to 82% alkali metal carbonates, from 15% to 35% alkali metal cyanides, from 3% to 15% anhydrous alkali metal silicates and up to 15% alkaline chlorides; it was indicated there that it is preferable that alkaline chlorides be present, preferably up to 10%, without however giving any explanation (it seems however that the presence of chlorides contributed to the preparation of the cyanides in a useable form). Moreover, there had been mentioned (see the document GB-1 052 668 published in 1966) carbonitriding baths, in crucibles having a composition within a well-chosen range, containing from 10 to 30% alkali metal cyanates and at least 10% alkali metal cyanides, at 600° C.-750° C.; a content of 25% alkali metal chloride was mentioned, with respect to a starting bath (moreover containing only cyanides (25%) and carbonates), as well as in the regeneration compound (containing moreover 75% cyanides). It has also been proposed (GB-1 185 640) to complete a carburizing stage with a short stage of dipping in a bath containing cyanides, cyanates, carbonates and alkali metal chlorides (without specifying ranges of contents for the latter).
For the nitriding of stainless steels, a gas phase nitriding treatment has been proposed (U.S. Pat. No. 4,184,899 published in 1980), preceded by a stage of thermochemical pretreatment in a bath containing from 4% to 30% cyanides and from 10% to 30% cyanates in combination with from 0.1% to 0.5% sulphur. It is mentioned that the remainder of the pretreatment baths can be formed by sodium carbonate or chloride, without these elements being active in the treatment (with respect to a bath with 12% cyanides and 0.3% sulphur, it is mentioned that at the start there is 25% sodium carbonate and 42.7% sodium chloride).
More recently, (see in particular the document U.S. Pat. No. 4,492,604 published in 1985) a nitriding bath has been proposed the cyanide content of which is comprised between 0.01% and 3%. It is indicated that, due to the strong reducing action of cyanides in nitriding baths at approximately 550° C.-650° C., whereas cyanates have a tendency to release oxygen, nitriding baths with a low cyanide content have a tendency to oxidize the nitriding layers and give rise to unacceptable coatings. In order to prevent the formation of such defects, it is taught to include up to 100 ppm of selenium, in combination with an appropriate composition of the crucibles (without iron).
It has also been proposed to harden ferrous parts using a bath containing high chloride levels (see the document EP-0 919 642 published in 1999), but this bath in fact serves to complete a nitriding action, being intended to allow the introduction of chromium (present in this bath in addition to the chlorides, with silica) into a previously formed nitriding layer.
For nitriding ferrous parts, a molten-salt bath has been proposed by the document U.S. Pat. No. 6,746,546 (published in 2004), containing alkali metal cyanates and alkali metal carbonates, with 45% to 53% cyanate ions (preferably between 48% and 50%), maintained between 750° F. and 950° F., i.e. between 400° C. and 510° C., in order to provide good corrosion resistance. The alkali metals were advantageously sodium and/or potassium (when both were present, the potassium content was preferably 3.9:1 relative to the sodium content); in operation, this bath contained from 1% to 4% cyanides (no details were given concerning the presence of any other elements in the bath).
Even more recently, in order to minimize the entrainment of the molten salts when the nitrided ferrous parts are taken out, the document U.S. Pat. No. 7,217,327 proposed a nitriding bath essentially constituted by Li-, Na- and K-type cations and by carbonate and cyanate anions.
It therefore appears that various compositions of molten salt baths have been proposed in order to allow nitriding of ferrous parts without using significant cyanide contents.
However, as a general rule, nitriding treatments with a low cyanide content (typically less than 3%,) must be followed by a finishing treatment because a low roughness is sought, which contributes to an increase in the cost of treatment (labour, grinding equipment) as well as the overall duration of treatment.
A low roughness can be obtained with nitriding baths with a high cyanide content (more than 5%), but after periods of several hours (typically 4 to 6 hours), which can appear too long on an industrial scale.